Conventional techniques for controlling charging properties of toners include incorporation of a charge control agent, e.g., nigrosine, into toner particles by melt-kneading.
It has been proposed to conduct charge control by adhering a polymer, such as an acrylonitrile polymer, a polymer having an aminophenylthio group, and a polymer of a diallylammonium halide, to the surface of the core of toner particles as disclosed in JP-A-62-106474 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-60-26347, and JP-A-60-26346.
Toners having a charge control function on the surface thereof further include those in which a vinylpyridine polymer or a vinylimidazole polymer is present on the surface of core particles, those in which a polymer of a compound having a tertiary amine in the molecule thereof is present on the surface of core particles, and those in which a polymer of (meth)acrylonitrile, (meth)acrylic acid, vinyl fluoride, methyl methacrylate, vinylcarbazole, vinylpyridine, dimethylaminoethyl methacrylate, diacetoneacrylamide, or N-vinylimidazole is present on the surface of core particles, as disclosed in JP-A-62-227161, JP-A-62-227162, JP-A-63-177145, JP-A-63-177147, and JP-A-63-177148.
Where a charge control agent is incorporated into toner particles, since it is only the agent present on the outer surface of toner particles that takes part in charge control, the agent must be used in large quantity.
On the other hand, where a charge control function is performed by a charge controlling polymer present on the surface of core particles, charging properties of the toner are dependent on the environmental conditions, particularly humidity because the monomer unit constituting the charge controlling polymer contains both a hydrophilic group and a charge control group per molecule. When a monomer containing both a hydrophobic group and a charge control group per molecule is used, environmental stability of the charging properties is improved, but the resulting toner shows a greatly broadened charge distribution.